One of the major objectives of the proposed work will be to examine the mechanisms responsible for the repression of BLV genome in vivo. Attempts will be made to clone the BLV genome and develop a fully representative molecular probe. We will use this probe to determine which cells contain BLV DNA and how much of the genome is present. We intend to characterize the structural proteins of BLV and to examine the role of their phosphorylation in the replication and maturation of the virus. We will also continue our studies using radioimmunoassays on the presence of BLV in milk and other excretions and secretions of cows. The role of these fluids in the natural mode of BLV transmission will be investigated using a sheep bioassay. Attempts will be made to characterize the functions of BLV induced lymphocyte subpopulations and to study their quantitative and qualitative changes in animals with high and low risks of lymphosarcoma.